[#2] Added Licensing.

I decided on the Apache-2.0 license and have stamped all the files to
now point to that license.

src/quaternion.rs

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: Apache-2.0
+// Sealed with Magistamp.
+
 //! This module defines the [Quaternion][1] and
 //! [DualQuaternion][2] structures.
 //!